1. Field of the Invention
The invention relates to components to be used in hip prostheses and, in particular, to acetabular cups and components therefor for use in hip prostheses and methods of making such components.
2. Discussion of the Related Art
In replacement hip surgery a femoral component is inserted into the prepared femur. The femoral component has a stem portion which projects into the femoral canal of the prepared femur and has an integral or separate modular head of substantially spherical shape. The ball-like head of the femoral component is received within an acetabular cup component which is implanted in the patient's hip socket, i.e, the acetabulum. The acetabular cup has a substantially hemi-spherical bearing surface for movement of the ball head of the femoral component during action of the joint. The acetabular cup is implanted into the prepared hip socket either with or without cement. Cementless types of acetabular cup may be secured in the prepared bone by a press fit or can be directly screwed in place or otherwise secured in place, for example by indirect means, e.g. by the use of separate bone screws passing through apertures provided in the acetabular cup. Generally, the femoral stem is of metal and the ball head is of metal or of a ceramic material.
Various designs of acetabular cups are available and it is often a multi-piece component having at least a separate outer shell and an inner liner. Where the acetabular cup has an inner liner, that inner liner is generally press-fitted into the outer shell. In some designs of hip prostheses the material of the bearing surface of the acetabular cup, e.g. its inner liner where present, is of the same material as that of the ball head, e.g. for a ceramic head a ceramic bearing surface is provided (a so-called ceramic-on-ceramic prothesis) and for a metal head a metal bearing surface is provided (a so-called metal-on-metal prothesis). In some other designs, the acetabular bearing surface is of polyethylene as the acetabular cup is either provided with a polyethylene inner liner or the acetabular cup is a single component made entirely from polyethylene. The shape of the bearing surface into which the ball head is received affects the degree of movement available after implantation of the joint.
It has been found that the use of polyethylene acetabular bearing surfaces in combination with metal or ceramic femoral ball heads leads to high wear rates of the polyethylene. Aseptic loosening of the separate components due to wear of the polyethylene is now emerging as a major factor in hip prosthesis longevity. As a result, there has been a resurgence of interest in alternatives to the use of polyethylene acetabular bearing surfaces. The wear rate is substantially reduced for metal-on-metal prosthesis and ceramic-on-ceramic prosthesis have been shown in most cases to give an extremely low wear rate and generate very little wear debris. There is, however, in a significant minority of cases the possibility of wear debris, e.g. ceramic or metal particles, becoming trapped between the articulating surfaces which can create further debris by third body wear. This sets up an accelerating spiral resulting in massive wear and loosening of the implant. One of the ways in which this can be initiated is impingement of the head or neck of the femoral component on the rim of the acetabular cup. This is particularly a problem with ceramic ball heads articulating with a ceramic acetabular cup. This can generate ceramic debris which gets between the articulating surfaces.
There is, therefore, a need for an improvement in acetabular cup design so as to provide a prosthesis having low wear characteristics.